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Introduction

When choosing the presentation for an analysis you will be required to enter a

value for the imaginary refractive index (this is effectively the absorption) of the

sample you are measuring. This can be difficult as the value has to be calculated

by performing an experiment. However, in most cases the value can be guessed

with very little effect on the result. In practice you will probably only use two

values; if the sample in transparent (glass beads for example) then there will be no

absorption so the value will be 0 (A on the grid), otherwise use 0.1 (H on the

grid) as the absorption value. If you feel you need a more accurate value then it

can be estimated by following the procedure below.

Estimating the absorption using

concentration measurements

The following technique may be useful in estimating the absorption in certain

cases. For a given measurement, the volume concentration is calculated using the

equation:-

where k is a constant (for fixed beam length and obscuration), V

and Q

are the

relative volume concentration and extinction efficiency for particles of diameter d

is sensitive to the optical properties and so, also, will be the concentration. The

technique then is first to determine the refractive indices of the particle and

medium either from tabulated values or by direct measurement using a

refractometer. A sample of known concentration is then prepared by mixing

weighed amounts of the materials and a measurement made. The data is analysed

using a range of presentations for the correct particle and medium refractive

indices and a range of absorptions. The presentation which gives the closest

agreement with the known volume concentration is then used as a good

approximation to the correct absorption value.

This technique has been used successfully with oil/water emulsions. It should be

noted that, a good estimate of the real refractive indices is necessary and the beam

length must be correctly entered (using the Setup - Hardware dialogue box),

therefore the method is difficult to apply with spray measurements.

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APPENDIX D

GETTING STARTED

Page D.1

Malvern Mastersizer X Series User Manual

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